Biomedical subjects
Publications and source records attributed to R M Epand.
Phosphatidic acid can replace phosphatidylserine in the activation of protein kinase C. However, in the presence of diolein, the addition of Ca2+ results in the inhibition of the enzyme. This phenomenon could lead to a negative feedback regulation of protein kinase C activity as a result of stimulation of the cycling of phosphatidylinositol.
Cholesterol sulfate is a component of several biological membranes. In erythrocytes, cholesterol sulfate inhibits hypotonic hemolysis, while in sperm, it can decrease fertilization efficiency. We have found cholesterol sulfate to be a potent inhibitor of Sendai virus fusion to both human erythrocyte and liposomal membranes. Cholesterol sulfate also raises the bilayer to hexagonal phase transition temperature of dielaidoyl phosphatidylethanolamine as demonstrated by differential scanning calorimetry and 31P nuclear magnetic resonance spectrometry. Although hexagonal phase structures are not readily found in biological membranes, there is a correlation between the effects of membrane additives on bilayer/non-bilayer equilibria and membrane stabilization. It is proposed that the ability of cholesterol sulfate to alter the physical properties of membranes contributes to its stabilization of biological membranes and the inhibition of membrane fusion.
Cholesterol sulphate is a potent stabilizer of membrane bilayer structure in both dielaidoylphosphatidylethanolamine and egg phosphatidylethanolamine model membranes, however, the addition of calcium abolishes this bilayer stabilization. Calcium also induces fusion and leakage of egg phosphatidylethanolamine large unilamellar vesicles containing cholesterol sulphate, but has no effect on fusion or leakage of egg phosphatidylcholine large unilamellar vesicles containing cholesterol sulphate. With egg phosphatidylethanoiamine liposomes, the initial rate, and extent of fusion, at constant calcium concentration, vary inversely with the mol percentage of cholesterol sulphate present in the vesicle membrane. The interaction of calcium and cholesterol sulphate, which causes membrane destabilization and fusion in phosphatidylethanolamine containing model systems, may play a role in the acrosome reaction in human sperm.
Leucine residues at positions 12, 16 and 19 of salmon calcitonin were systematically replaced by alanine either one, two or three at a time. Substitution of Ala at positions 12 or 16 had the greatest effect on the structure of the peptide and on the ability of the peptide to attain structures of higher helical content in the presence of lipid. Despite the similar effects on the conformational properties, the effects on activity are markedly different for analogs with Ala substitutions at positions 12 and 16. The Ala12 derivatives are much less active while the Ala16 derivatives are slightly more active than the parent hormone. In contrast to the effects of substitutions at these positions, substitution of Ala at position 19 has relatively little effect on activity or on conformation. These results demonstrate that different regions of the calcitonin molecule have different conformational requirements for maximal activity.
The effect of human parathyroid hormone-(1-34) (hPTH) and human calcitonin (hCT) on the activity of the Ca2(+)-extrusion pump in liver plasma membranes was studied. Both hormones were found to be potent inhibitors of Ca2+ transport and the related high-affinity (Ca2(+)-Mg2+)-ATPase activity, causing maximal inhibition of 25-30% at concentrations of 100 nM. Half-maximal inhibition was observed with 20 nM-hPTH and with 0.5 nM-hCT. By comparison, salmon calcitonin and intact bovine parathyroid hormone-(1-84) were inhibitory only at 10 microM. The effects of hCT and hPTH on the Ca2+ pump activity were not mimicked by cyclic AMP. Also, 10 microM of either hPTH-(1-34) or hCT did not alter the 45Ca2+ influx rate into isolated hepatocytes. We conclude that inhibition of Ca2+ efflux, rather than the stimulation of Ca2+ influx, may play a functional role in the control of hepatic calcium homeostasis by hPTH-(1-34) and hCT.
Retinol has little effect on the activity of protein kinase C. However, air oxidation of retinol produces products which are inhibitory to this enzyme. These results are consistent with a suggestion that the activity of protein kinase C is modulated by the bulk biophysical properties of its environment. The facile susceptibility of retinol to oxidation, as demonstrated by HPLC analysis, explains some of the discrepant reports of its effects on the activity of protein kinase C.
Under physiological conditions, protein kinase C is active when bound to membranes. Like most membrane-bound enzymes, its activity is dependent on the nature of its lipid environment. In this article, Richard Epand and David Lester describe the relationship between the ability of specific membrane-active agents to alter the biophysical properties of the lipid environment and their potential to modulate protein kinase C activity. They argue that this can lead to a greater understanding of the mechanism of inhibition and activation of protein kinase C through modulation of the bulk biophysical properties of the membrane, and may provide a new approach to the development of a more specific set of inhibitors.
The intracellular pH of a number of multidrug resistant cell lines was compared with that of their parental lines using the fluorescent probe bis-carboxyethylcarboxyfluorescein. In four different cases, cells having 5-fold resistance or more exhibited an intracellular pH which was 0.10-0.17 units higher than that of the parental cell line. A CHO cell line, AB1, and its 180-fold resistant counterpart, CHRC5, were further investigated with regard to the role of Na+/H+ antiport. The Na+/H+ antiport activity was greater at any intracellular pH for the CHRC5 cells than the AB1 cells. To investigate the possible role of higher intracellular pH in multidrug resistance, the effect of several agents which are either known to reverse multidrug resistance or inhibit Na+/H+ antiport activity were examined. Verapamil was found to reverse multidrug resistance but had no effect on intracellular pH while amiloride, which acidifies the cytoxol by blocking Na+/H+ antiport activity, did not cause reversal of drug resistance. In contrast to verapamil, treatment of CHRC5 cells with cyclosporin A had a parallel effect on reversal of their drug resistant phenotype and a lowering of their intracellular pH to that of the sensitive cell level. However, cyclosporin was ineffective in either lowering the intracellular pH or reversing drug resistance in DC3F/ADX cells. Therefore, except for the effect of cyclosporin A on the CHRC5 line, the effects of other agents on reversal of multidrug resistance and intracellular pH did not correlate with each other.
Different epitopes residing within the decapeptide of residues 80-89 of human myelin basic protein (MBP) exist in the MBP-like material detected in human CSF and urine. In the present study, the structure of human MBP peptide 80-89 was examined by a combination of physical measurements and correlated with its varying immunochemical reaction with three polyclonal antisera. At least two epitopes are present in the decapeptide. Progressive shortening and reduction in net negative charge of MBP peptide 80-89 to form peptides 81-89, 82-89, 83-89, and 84-89 revealed an epitope not present in intact MBP. Circular dichroism and Fourier-transform infrared of these MBP peptides in water demonstrated random structure that was partially changed to beta-structure in the shorter peptides. In methanol, used as a model for a lipid environment, the random structure was diminished and was replaced by alpha-helix and beta-structure, especially in the shorter peptides. The findings indicate that the range of epitopes present in this decapeptide is influenced by conformation, which, unexpectedly, becomes progressively less random as the peptide becomes smaller, especially in a hydrophobic environment. This behavior has implications for the immunochemical detection of small antigens or antibodies to them in tissue extracts or body fluids.
Phospholipid bilayers can undergo morphological rearrangements to other phases. The formation of one of these nonbilayer phases, the hexagonal phase, is preceded by an increase in the hydrophobicity of the bilayer surface and a destabilization of the bilayer structure. Certain membrane additives promote, while others inhibit, the formation of the hexagonal phase. Many of the molecular features that determine this phase preference are understood. Some of the properties of membranes are modulated by agents that affect the relative stability of the bilayer and hexagonal phases. Addition of bilayer stabilizers to a membrane decreases its fusogenic behaviour. One such bilayer stabilizer is cholesterol sulfate, which may function physiologically to inhibit the fusion of sperm cells. Several antiviral agents are also found to be bilayer stabilizers and some have been shown to inhibit membrane fusion phenomena. Another biological property that is modulated in a predictable manner by agents which affect the bilayer-hexagonal phase equilibrium is insulin-promoted glucose uptake in adipocytes. Bilayer stabilizers inhibit this process showing that the effects of insulin can be modulated by the bulk biophysical properties of the membrane. The activity of a number of membrane-bound enzymes is also lowered by bilayer stabilizers. Neutral and zwitterionic bilayer stabilizers are inhibitors of protein kinase C. Thus, the alteration of the bilayer-hexagonal phase transition by drugs may provide a useful parameter for predicting their effects on biological membranes.
Binding of the nonhelical salmon calcitonin (sCT) analog, [Gly8,Ala16]-des-Leu19-sCT to membrane preparations from rat brain could be analyzed in terms of two independent binding sites. The high and low affinity binding sites for this analog were named CT-L (L, linear) and CT-H (H, helix), respectively. Although the CT-H type receptor has a low affinity for the nonhelical analogs, it binds the helical sCT with high affinity and therefore represents a CT binding site. The physiological significance for the existence of subtypes of specific CT receptors is not clear. The [Gly8,Ala16]-des-Leu19-sCT suppressed the osteoclastic bone resorption in tissue culture at low concentration (0.1 nM). The dose of [Gly8,Ala16]-des-Leu19-sCT required for this hypocalcemic activity was highly correlated with the binding affinity of this analog to the CT-L receptor subtype. In addition, human CT interacted with the CT-L type receptor at about 100th the concentration of that required for the displacement of sCT. We conclude that binding to the CT-L type receptor is required for hypocalcemic activity in mammals.
Platelets from diabetic patients are hypersensitive to agonists in vitro. Membrane fluidity modulates cell function, and reduced membrane fluidity in cholesterol-enriched platelets is associated with platelet hypersensitivity to agonists, including thrombin. Decreased membrane fluidity of these platelets is attributed to an increased cholesterol-phospholipid molar ratio in platelet membranes. We examined the response of platelets from diabetic subjects to thrombin, platelet membrane fluidity, and platelet cholesterol-phospholipid molar ratio. Twelve poorly controlled diabetic subjects were compared with 12 age- and sex-matched control subjects. In response to a low concentration of thrombin, mean values for release of [14C]serotonin from washed prelabeled platelets were not significantly different between diabetic and control subjects, but in 8 of 12 diabetic subjects, the release response was greater than in their paired control subjects. Mean steady-state fluorescence polarization values in 1,6-diphenyl-1,3,5-hexatriene-labeled platelets prepared from diabetic subjects were significantly greater than in control subjects; this indicates a decreased membrane fluidity in platelets from diabetic subjects. Total or very-low-density (VLDL), low-density (LDL), or high-density (HDL2, HDL3) lipoprotein cholesterol concentrations in plasma were not significantly different between groups; however, the ratio of VLDL + LDL to HDL2 + HDL3 was significantly greater in diabetic than in control subjects. There was no difference in the total platelet cholesterol-phospholipid molar ratio between groups.(ABSTRACT TRUNCATED AT 250 WORDS)
Several series of amphiphiles of increasing chain length were tested for their abilities to modify the L alpha-HII transition of dielaidoylphosphatidylethanolamine using differential scanning calorimetry. Acylcarnitines, alkyl sulfates, alkylsulfobetaines, and phosphatidylcholines, with chain lengths between about 6 and 12 carbon atoms, show an increasing capacity to raise the L alpha-HII phase transition temperature of phosphatidylethanolamine. This is ascribed to increased partitioning of the added amphiphile from water into the membrane as the chain length increases. Alkyl sulfates and alkyltrimethylammonium bromides have diminished capacities to raise the L alpha-HII transition temperature as the chain length is increased from 12 to 16. This is caused by an increase in the hydrophobic portion of the amphiphile leading to a change in the intrinsic radius of curvature and a decrease in the hydrocarbon packing constraints in the HII phase relative to the shorter chain amphiphiles. The L alpha-HII transition temperature of phosphatidylethanolamine with acylcarnitines of chain length 14-20 carbon atoms, alkylsulfobetaines above 14 carbon atoms, and phosphatidylcholines with acyl groups having above 10 carbon atoms is relatively insensitive to chain length. We suggest that this is caused by a balance between increasing hydrocarbon volume promoting the HII phase through decreased intrinsic radius of curvature and greater relief of hydrocarbon packing constraints vs greater intermolecular interactions favoring the more condensed L alpha phase. This latter effect is more important for amphiphiles with large headgroups which can pack more efficiently in the L alpha phase. The phosphatidylcholines show a gradual decrease in bilayer stabilization between 10 and 22 carbon atoms.(ABSTRACT TRUNCATED AT 250 WORDS)
The phase behavior of mixtures of cholesterol or epicholesterol with phosphatidylethanolamine was studied by differential scanning calorimetry and by X-ray diffraction. Discrete domains of cholesterol are detected by X-ray diffraction in the L alpha phase of phosphatidylethanolamine from egg yolk and synthetic dielaidoylphosphatidylethanolamine beginning at mole fractions of 0.35-0.4 cholesterol. Separate domains of crystalline epicholesterol can also be detected in the L alpha phase of dielaidoylphosphatidylethanolamine by X-ray diffraction at as little as 0.16 mole fraction of epicholesterol. This is a result of poor miscibility of the epicholesterol with dielaidoylphosphatidylethanolamine. Epicholesterol does not alter the L beta----L alpha transition or bilayer spacing. Epicholesterol also has little effect on the diameter of the cylinders in the hexagonal phase. Formation of the inverted hexagonal phase is facilitated by addition of small amounts of cholesterol (mole fraction less than 0.2) in both egg phosphatidylethanolamine and dielaidoylphosphatidylethanolamine. However, at higher mole fractions of cholesterol, the stability of the liquid-crystalline phase is found to increase markedly for dielaidoylphosphatidylethanolamine but not for egg phosphatidylethanolamine, indicating the importance of the structure of the acyl chains in controlling the relative stability of the lamellar and nonlamellar phases in these systems. In contrast to cholesterol, epicholesterol markedly lowers the L alpha----HII phase transition temperature at low mole fraction of sterol. This result demonstrates the importance of the orientation and motional properties of an additive in determining the L alpha----HII transition temperature.
The uptake of labeled palmitoyl carnitine and palmitoyl lysophosphatidylcholine by CHO cells was studied by measuring the extractability of these amphiphiles by bovine serum albumin. A multidrug-resistant cell line, CHRC5, showed a more rapid uptake, compared with the parental line, of these amphiphiles into a pool that was no longer susceptible to extraction with bovine serum albumin. The more rapid uptake by the drug-resistant cell line was reversed back to the rates observed with the parental cell line in the presence of verapamil, quinacrine or cyclosporin A. These latter three drugs also reverse the multidrug-resistant phenotype. These results demonstrate a relationship between the rate of amphiphile uptake and multidrug resistance.
The role of the 2-amino group of sphingosine on the in vitro inhibition of protein kinase C was investigated by comparing protein kinase C activity in the presence and absence of sphingosine at various pH's. Inhibition by sphingosine was found to be pH dependent. Above pH 7.75, sphingosine has little or no inhibitory effect. In fact, at pH 8.5, sphingosine slightly enhances enzyme activity above that which occurs when the enzyme is stimulated by diacylglycerol and phosphatidylserine. After correcting for electrostatic repulsion, we find that the intrinsic pK for sphingosine in Triton micelles is 8.5. Inhibition of protein kinase C by sphingosine at physiological pH's therefore correlates with the presence of a positive charge.
Human myelin basic protein (MBP) was fractionated into several of its charge isomers (components). Of these, the secondary structures of four isomers before and after phosphorylation have been studied by circular dichroism (CD). None of the four showed any alpha-helical structure. All of the components showed varying amounts of beta-structure, random structure, and turns. Component 1 (C-1), the most cationic of the components, showed 13%; component 2 (C-2) had 19%; C-3, 17%; and C-4, 24% of beta-structure. Each of the four components was phosphorylated with protein kinase C, from human brain. The extent of phosphorylation varied considerably from 2.8 +/- 0.6 mol of PO4/mol of protein in C-1 to 5.2 +/- 0.8 mol of PO4/mol of protein in C-4. The effect of phosphorylation on the secondary structure was to induce beta-structure in all the components. The largest change in beta-structure was in C-1 and the least in C-4. The surprising result is that although the components were phosphorylated to different extents, the amount of beta-structure in all four components increased to a final proportion of 35-40%. Treatment of phosphorylated C-1 with acid phosphatase removed 50% of the total radioactivity. Although the remainder represented approximately 1 mol of PO4/mol of protein, the proportion of beta-structure was unaltered. We concluded that a single phosphorylation site identified as residues 5-13 represented a critical size for stabilization of beta-structure of MBP in solution and that phosphorylation at the other sites had little influence on secondary structure.
The peptides [Glu1,8,Leu11,17] 18A and [Glu4,9,Leu11,17] reverse-18A are 18-residue peptides designed to form amphipathic helices with opposite charge distribution; [Glu1,8,Leu11,17] 18A having positively charged residues at the hydrophobic/hydrophilic interface. Both [Glu1,8,Leu11,17] 18A and [Glu4,9,Leu11,17] reverse-18A strongly disrupt the bilayer structure as indicated by the relatively narrow lipid 1H and 31P NMR peaks. In addition, the 1H chemical shift of the quaternary ammonium methyl groups indicates that [Glu1,8,Leu11,17] 18A forms smaller lipoprotein particles with dimyristoylphosphatidylcholine (DMPC) than does [Glu4,9,Leu11,17] reverse-18A. However, motional properties of the lipid head group indicate that no specific salt bridges are formed between the phospholipid head group and the side chains of polar amino acids of either of the two peptides. In addition, the acyl chain conformation for the DMPC complexes with [Glu1,8,Leu11,17] 18A and with [Glu4,9,Leu11,17] reverse-18A are indistinguishable by the criterion of IR spectroscopy. The 2H linewidth of the solvent 2H2O remains narrower in frozen solutions of the DMPC-[Glu1,8,Leu11,17] 18A complexes suggesting the presence of more unfrozen bound water in this case. The two peptides exhibit many similarities in their interaction with lipids. However, [Glu1,8,Leu11,17] 18A can more readily lyse vesicles and activate lecithin:cholesterol acyltransferase. These differences do not appear to result from differences in specific charge interactions between the lipid and peptide but may be manifested through differences in hydration properties.